The dye laser has become the dominant tool in the field of tunable laser spectroscopy. Its wide continuous tuning range, from the ultraviolet to the infrared, offers a potential unequaled by any other source. This very tunability, however, makes narrow-band operation difficult since one optical element (grating, prism, wedge filter or etalon) is required to tune within the operating range of any dye (typically 20-50 nanometers). With this single element, band-widths down to 0.01 nm have been achieved but more typical values are .05-2 nm. Further narrowing requires an additional element, usually an etalon, requiring that two wave-length selecting elements be tuned synchronously to scan the laser. Furthermore, longitudinal cavity modes become important in some lasers so that cavity length also must be varied.